The present invention is a voltage controlled oscillator (VCO) designed to automatically compensate for processing, temperature and voltage supply variations. A patent assigned to National Semiconductor Corporation and issued to Rasmussen, U.S. Pat. No. 5,061,907, had similar goals, but achieves process, temperature and voltage supply immunity using somewhat different techniques from the present invention.
In summary, Rasmussen discloses a VCO that includes a multistage ring oscillator, a voltage-to-current converter, process compensation circuitry, and a trip-point compensation circuit. The voltage-to-current converter linearly converts the input signal (typically from a phase-locked loop filter) to an output current signal that is independent of the transistors in the ring oscillator, i.e., independent of varying temperature and process. Thus, the linear voltage-to-current converter in Rasmussen provides a variable control current rather than a constant current source as used by the present invention. The process compensation circuitry in Rasmussen responds to the tuning voltage input signal to provide a current dump output signal that is dependent on transistor strength, i.e., dependent on temperature, process and supply voltage. The trip-point compensation circuit in Rasmussen generates a net current created by subtracting the process compensation current from the current generated by the voltage-to-current converter, and then steers a variable percentage of that net current to the ring oscillator in accordance with the strength of the N- and P- channel transistors in the device.
The output currents from both the voltage-to-current converter and the process compensation circuitry in Rasmussen are dependent on the tuning voltage input signal. The dependence of the process compensation circuitry on the tuning voltage input signal, however, can create an unstable current dump output signal. The present invention avoids this dependence by providing a current source and a process compensation circuit that are not dependent on the tuning voltage input signal.
Thus, it is an object of the present invention to provide a VCO where process spread, temperature and supply voltage variation has minimal effect on output frequency and the open loop gain of the VCO.